encoding/json: properly handle extra objects with fixed size arrays

If a fixed size array is passed in as the decode target and the JSON
to decode has extra array elements that are objects, then previously
the decoder would return a "data changing underfoot" error.

Fixes #3717.

R=golang-dev, adg, rsc
CC=golang-dev
https://golang.org/cl/7490046

2 files changed, 23 insertions, 0 deletions

diff --git a/src/pkg/encoding/json/decode.go b/src/pkg/encoding/json/decode.go
index d34e10f838..722037c309 100644
--- a/src/pkg/encoding/json/decode.go
+++ b/src/pkg/encoding/json/decode.go
@@ -261,6 +261,16 @@ func (d *decodeState) value(v reflect.Value) {
 		}
 		d.scan.step(&d.scan, '"')
 		d.scan.step(&d.scan, '"')
+
+		n := len(d.scan.parseState)
+		if n > 0 && d.scan.parseState[n-1] == parseObjectKey {
+			// d.scan thinks we just read an object key; finish the object
+			d.scan.step(&d.scan, ':')
+			d.scan.step(&d.scan, '"')
+			d.scan.step(&d.scan, '"')
+			d.scan.step(&d.scan, '}')
+		}
+
 		return
 	}
 
diff --git a/src/pkg/encoding/json/decode_test.go b/src/pkg/encoding/json/decode_test.go
index e1bd918dda..037c5b2368 100644
--- a/src/pkg/encoding/json/decode_test.go
+++ b/src/pkg/encoding/json/decode_test.go
@@ -1178,3 +1178,16 @@ func TestUnmarshalJSONLiteralError(t *testing.T) {
 		t.Errorf("got err = %v; want out of range error", err)
 	}
 }
+
+// Test that extra object elements in an array do not result in a
+// "data changing underfoot" error.
+// Issue 3717
+func TestSkipArrayObjects(t *testing.T) {
+	json := `[{}]`
+	var dest [0]interface{}
+
+	err := Unmarshal([]byte(json), &dest)
+	if err != nil {
+		t.Errorf("got error %q, want nil", err)
+	}
+}